For turbine rotor blades and particularly high pressure (HP) turbine blades, there is an industry wide and ever-important objective to minimise both over-tip leakage (OTL) of hot working gases between a tip of the blades and a casing and heat transfer from the hot working gases to the blade. OTL occurs because of the pressure differential between a pressure-side and a suction-side of a turbine blade; this pressure differential can be referred to as ‘driving pressure’.
In general, there are three types of tip geometry configurations which attempt to minimise over tip leakage: un-shrouded, partially shrouded (or ‘winglet’) and fully shrouded. The simplest form of tip geometry is an un-shrouded type having a flat tip (see FIG. 1). A flat tip design is typically associated with a relatively high aerodynamic loss due to the over-tip leakage flow and high heat transfer, although it is relatively simple to manufacture. Other configurations have been developed with the main intent to reduce the over-tip leakage flow and losses. One such type is called a ‘squealer’ (FIG. 2), which has pressure and suction walls sealed with a tip plate and ribs or fins extending from the tip plate to define a tip cavity. Another blade tip design is referred to as a ‘winglet’ (FIG. 3), which is effectively a partially shrouded blade and also has ribs or fins extending towards the casing.
Both the squealer and winglet designs form a tip cavity that serve to avoid losses in efficiency by reducing the amount of leakage flow passing over the tip and reducing the flow disturbances set up by the leakage flow. The gas that passes over a pressure side fin of the cavity forms a vortex in the cavity. Whereas these blade designs help to prevent over-tip leakage, they both require substantial amounts of cooling air. In particular, the over-tip leakage forms a vortex which impinges on the suction side of the blade causing significant heat transfer. The amount of cooling is largely determined by the heat load on the blades the hot mainstream gases.
The present invention therefore seeks to minimize over tip leakage and reduce heat load from the working gas into the blades.